In an image forming device such as a copier and a printer, a toner agent (a developing agent) is transferred to a paper sheet to form (print) an image on the paper sheet. More particularly, in an image forming device, a transfer device to which transfer voltage is applied and transfer current is supplied transfers a developed image (toner image) carried on an image carrier (photoconductor) to a paper sheet, whereby printing is carried out.
In such an image forming device, images can be printed on both a front surface and a back surface of a paper sheet. However, in both-sided printing, transfer properties of toner images to the paper sheet are different between an image printed on one surface (hereinafter, referred to as a first side) of a paper sheet and an image printed on the other surface (hereinafter, referred to as a second side) of the paper sheet. This may cause a problem that the image printed on the first side and the image printed on the second side are not the same in quality.
FIG. 12 is a graph showing a change in a moisture content on surfaces of a paper sheet during both-sided printing on the paper sheet. FIG. 13 is a graph showing a relationship between a moisture content on a surface of a paper sheet and a surface resistance value of the paper sheet. A reason for the occurrence of the above problem will be described with reference to FIGS. 12 and 13. That is, when printing an image on a paper sheet, a fixing device heats to fuse toner that has been transferred to the paper sheet, and causes the toner to be fixed to the paper sheet. At this time, when heated by the fixing device, a moisture content on the surface of the paper sheet evaporates and, as illustrated in FIG. 12, the moisture content on the surface of the paper sheet after a fixing process is lowered significantly. Therefore, the moisture content in the paper sheet when a transfer process is performed to the second side is significantly lower than the moisture content in the paper sheet when the transfer process is performed to the first side. Here, as illustrated in FIG. 13, it is known that the lower the moisture content on the surface of the paper sheet becomes, the higher the electrical resistivity on the surface of the paper sheet becomes. Not using a transfer voltage in accordance with the electric resistivity on the surface of the paper sheet may cause poor transfer of toner to the paper sheet by the transfer device, that is, uneven density and lack of applied toner. As a result, image reproducibility is impaired. Therefore, when the transfer process is to be performed to the second side of which moisture content has changed, if the transfer is performed under the same transfer condition as in the transfer process in the first side, the image printed on the first side and the image printed on the second side are not the same in quality.
In view of the above problem, a technique to make a difference in quality of images printed on the first side and the second side smaller is disclosed in PTL 1.
In the image forming device disclosed in PTL 1, a voltage to be applied to a transfer device can be switched into two transfer voltages. In the image forming device, the transfer voltage in printing on the second side is set to be larger than the transfer voltage in printing on the first side, whereby a difference in quality between the image printed on the first side and the image printed on the second side is made to be smaller.